How can I use my Blender models in WebGL
The answer is somewhat dependent on what file format the WebGL viewer ingests. And how good the Blender exporter is. But the most common format is probably the ASCII Obj format, for which Blender has a pretty decent exporter for.

So, things to watch out for:

Geometry: Avoid Ngons, or at least, triangulate before exporting. The exporter might produce unexpected results. Also, make sure the face normals are all oriented correctly. Blender renders both side of a polygon by default, many other 3D viewers don't. You can recalculate normals with Ctrl + N.

Mesh Modifiers: Even though the exporter probably applies the modifiers. It's better to be explicit than implicit. Also, it is a good idea to apply the transforms (Ctrl + A).

Materials and Textures: Avoid the Blender material panel(except basic colors). Use UV-mapped textures. Blender-specific features like procedural textures are not going to migrate properly. Image textures dimensions should be power of 2 square. (1024x1024, 512x512, etc)